A head-up display device disclosed in Patent Literature 1 is known as a conventional head-up display device. The head-up display device comprises a display device, a collimator lens, and a pair of plane mirrors arranged in parallel. One of the pair of plane mirrors is a semi-transparent mirror that reflects a part of incident light and transmits a part thereof. Light (image light) transmitted from the display device enters the collimator lens and becomes parallel light, and enters a pair of plane mirrors arranged in parallel. The parallel light entered into the pair of plane mirrors arranged in parallel repeats reflection between the plane mirrors. As one of the plane mirrors arranged in parallel is a semi-transparent mirror, a part of the parallel light entered into the semi-transparent mirror is emitted from the semi-transparent mirror. The parallel light emitted from the semi-transparent mirror is reflected by a plane transparent plate (so-called a combiner), and the parallel light reaches an eye of an observer. As the parallel light enters an eye of an observer, an observer recognizes as if a display image is present in a distant place by viewing a virtual image projected onto the combiner.